What they sometimes do in certain sheep-raising nations is to "brake" a leg. This means they attach a clog or weight to the animal's leg, which keeps certain "rogue" sheep from getting too far from the shepherd until they learn their names, and not to be afraid of the shepherd.
Rogue sheep are those that won't stay with the flock--important to their safety. A single sheep that constantly moves out and away from the others is the certain target of predators, and often is at risk of wandering out of sight (over a hill, into the brush, etc.) in terrain where the shepherd is unable to count the sheep properly. Then the sheep would be surely lost.
Each shepherd looks after from about 1200 to 3000 sheep. When they're constantly moving, such large numbers are impossible to count with precision.
To keep track of such large numbers of sheep, they must be corralled, and then "passed under the rod," which means the shepherd has them in a narrow chute that enables each sheep to be counted one by one, and even marked with paint, charcoal, etc., for further work if necessary.
The leg brake is a temporary measure; a lamb with a braked leg (it's not a "broken" leg!) is still easy pickings for predators at night, because it can't run as fast as the flock when under attack, and shepherd usually can't see predators in the dark.
